This invention relates to nuclear fuel assemblies, and in particular to an arrangement for retaining nuclear fuel rods in the proper spaced relationship within the assembly.
Fuel assemblies for pressurized water nuclear reactors typically have a lower end fitting to which is connected an elongated frame for vertically and laterally supporting a multiplicity of parallely spaced nuclear fuel rods. The frame includes a plurality of axially spaced grids, each grid defining a multiplicity of openings which are in registry with one another over the longitudinal extent of the fuel assemblies. Each fuel rod passes through an opening in each grid, and is spaced and supported within the grid by resilient means projecting from the grid walls into the openings and against the fuel rod. During operation, a nuclear reactor core typically contains over two hundred fuel assemblies arranged side-by-side so that the lower end fittings rest on a core support stand.
Cooling water is introduced into the lower ends of the assemblies through the lower end fitting. As the water passes upward along the assemblies, a net upward force is applied to the individual fuel rods. This coolant flow over the rods not only tends to lift them upwardly, but also may cause vibrations to begin in the lower, or upstream, end of the rods. Measures must be taken to prevent the fuel rods from vertically rising through the grids and being damaged upon impact against other structures above the assemblies, in the event of a major system malfunction such as a loss of coolant accident. These measures must not be permanent, however, since it is desirable that a fuel assembly be remotely reconstitutable. This requires that individual fuel rods be removable from the assembly in the event that leakage or other defects are encountered during the refueling period but prior to the scheduled permanent removal of the entire fuel assembly.
Furthermore, every effort must be made to minimize the neutron poison effect of structural material in the assembly, and accordingly the type and location of the structure are important considerations. Moreover, materials compatibility is also important particularly in fuel assemblies where the fuel rods are made of Zircaloy and the grids or resilient members on the grids are made from a different material such as Inconel or stainless steel.